Fix required CMake version

[Flamefire]

cxx_std_11 requires CMake 3.8

[Lastique]

cxx_std_11 is not used in Boost.Filesystem. Why is this needed?

[Flamefire]

It should be used because the library requires C++11, see the Jamfile:

filesystem/build/Jamfile.v2

Lines 132 to 142 in ce7a835

	cxx11_rvalue_references
	cxx11_scoped_enums
	cxx11_noexcept
	cxx11_nullptr
	cxx11_defaulted_functions
	cxx11_defaulted_moves
	cxx11_deleted_functions
	cxx11_function_template_default_args
	cxx11_unified_initialization_syntax
	cxx11_final
	cxx11_override

It doesn't fail without it though because this gets added by a dependency already which is what causes the error in the dependency and Boost.Filesystem.

[Lastique]

It should be used because the library requires C++11, see the Jamfile

This is reflected in CMake here:

filesystem/CMakeLists.txt

Lines 78 to 89 in ce7a835

	target_compile_features(boost_filesystem PUBLIC
	cxx_rvalue_references
	cxx_strong_enums
	cxx_noexcept
	cxx_nullptr
	cxx_defaulted_functions
	cxx_defaulted_move_initializers
	cxx_deleted_functions
	cxx_default_function_template_args
	cxx_final
	cxx_override
	)

I believe, the dependency that requires it is Boost.Atomic, but that got recently fixed. Do you still have the issue after that fix was applied?

[Flamefire]

I missed that, sorry.

I believe, the dependency that requires it is Boost.Atomic, but that got recently fixed. Do you still have the issue after that fix was applied?

Do you mean boostorg/atomic#78 ? That means to build Boost.Atomic you (declare to) need CMake 3.8, and as FileSystem requires Atomic it needs CMake 3.8 too.

The error here shows because cxx_std_11 is added to boost_filesystem by boost_atomic so CMake shows the error for both targets.

You can also simplify the list to just cxx_std_11 as you'll get that anyway.

[Lastique]

The thing is if CMake < 3.8 is used to build Boost.Filesystem, it should fail at Boost.Atomic, as expected. There's nothing in Boost.Filesystem itself that requires 3.8, so it should not be an error to specify 3.5. Projects should not be required to track the max min required CMake version across their dependency tree, it's CMake's job to figure that out. I consider this a CMake issue.

Anyway, raising the min version is not really a problem, if it works around the issue. Thanks again.

[Flamefire]

The thing is if CMake < 3.8 is used to build Boost.Filesystem, it should fail at Boost.Atomic, as expected.

With boostorg/atomic#78 it now does. It didn't before and I simply opened PRs in each repo where CMake showed the unknown feature w/o checking if that feature was used by the library itself or a dependency as I wasn't aware it would verify features so late the error shows up in dependents too.

With "as FileSystem requires Atomic it needs CMake 3.8 too." I didn't want to say that you need to declare that you need CMake 3.8 as done in this PR but that you effectively do require CMake 3.8 because otherwise configuring Filesystem will fail while configuring the Atomic dependency.
So there is no real change by changing the minimum version here.

But: As you effectively need CMake 3.8 anyway you can replace that long list with cxx_std_11 and directly declare the 3.8 requirement as done in this PR.